1. Field of the Invention
The present invention relates to a method of activating a zirconia oxygen sensor which detects the oxygen concentration of an ambient atmosphere by means of a zirconia element that has a porous electrode formed on both sides of an impervious oxygen ion conductor.
2. Description of the Related Art
The sensing element used in zirconia oxygen sensors is generally formed of a zirconia thimble having an inner and outer metal coating, usually platinum, to form an electrode. The electrode is then used to measure the differential oxygen concentration between the measured gas on the outside of the thimble, and a reference gas, usually atmospheric, on the inside of the thimble. By measuring the voltage between two electrodes, the differential oxygen concentration can be calculated.
Solid electrolyte oxygen sensors comprising of gas impermeable zirconia ceramic separating two conductive (Pt) electrodes are widely used for combustion control in power plants as well as in the exhaust of automotive internal combustion engines. For utilization of oxygen sensors for industrial combustion control, the sensor must demonstrate certain performance criteria, i.e. a typical relative accuracy of between 3-5% (or absolute accuracy of 0.1-0.2%), a response time of less than 10 seconds, and a life expectancy typically greater than 1 year.
Oxygen Sensors used for automotive applications (Exhaust Gas Oxygen (EGO)) usually require different set of performance criteria. Beginning with 1994 model year, automotive manufacturers have been required to implement on board diagnostic systems, which can diagnose and detect malfunctioning emission related components. These systems usually involve dual exhaust gas oxygen sensors. One sensor is placed directly behind engine exhaust, and a second sensor is placed downstream of the three-way catalyst. The exhaust gas oxygen sensor generates large (−800-1,000 mV) voltage outputs in response to cyclic fluctuations in air/fuel ratio about the stoichiometric point caused by closed loop fuel control of the engine.
Desirable characteristics of the oxygen sensor placed directly behind the engine for an optimum close-loop control of inlet air-fuel mixture are high voltage output, fast switching time and reasonably long longevity (>100,000 miles). A Second sensor is, placed behind the three-way catalytic converter and exhibits reduced voltage output due to an increase in excess oxygen present in the exhaust stream which reflects oxygen storage capacity of the catalyst thus allowing evaluation of the hydrocarbons conversion efficiency. It is known that the presently used method can detect only a very narrow range of catalyst efficiencies. Below 80-90% conversion efficiencies, the rear EGO sensor index tends to saturate and provides little monitoring capability.
This sensor can greatly benefit from possessing performance characteristics typical for industrial oxygen sensors, which can accurately measure excess oxygen directly—rather than extract excess oxygen values through elaborate and indirect data processing typical for dual exhaust gas oxygen sensors systems. However, elaborate and expensive manufacturing methods used for industrial oxygen sensors manufacturing makes direct implementation of these sensors in automotive market cost prohibitive.
In the related art is known several improvements, and drawbacks, in preparing the sensing element. For example, it is known that structures that improve the diffusion of oxygen through the sensor electrodes can increase the accuracy and longevity of the completed sensor, while impediments to oxygen diffusion decrease the accuracy or longevity of the completed sensor. Currently, the best known improvements to aid in the diffusion of oxygen through the sensor electrodes include: to somehow create a distributed “microporosity” throughout the sensor electrode to provide efficient channels for diffusion; and increasing zirconia-platinum adhesion to similarly lessen any interface resistance that can affect impedance. Alternately, if oxidizing conditions exist at the zirconia-platinum interface during manufacturing an oxide layer of platinum is known to form. This oxide layer contributes to weak platinum adhesion to the zirconia substrate, and increased interface resistance. Additionally, high temperatures, required for sensor manufacturing, lead to electrode sintering, causing imperviousness to gas oxygen, thus increasing sensor impedance and deteriorated performance
Numerous attempts have been made to generate the foregoing improvements. For instance, U.S. Pat. No. 5,433,830, issued in the name of Kawai et al., discloses a method for activating a zirconia oxygen sensor by heating the zirconia sensor body and attached platinum coating assembly in a temperature-controlled furnace, while applying an alternating treating voltage. As a result, the treatment current flowing through the sensor can activate the sensor by causing physical changes to occur in the Pt electrodes, and improving electrode porosity. Applied voltage (generated current) was specifically limited to below the reduction potential to prevent reduction of zirconia (blackening). Bulk reduction of zirconia is indeed detrimental to its performance due to potential cracking. However, surface reduction of zirconia can be beneficial for oxygen sensor performance, since it removes the oxide layer from Pt and zirconia and allows direct contact between Pt and Zirconium metal. Metal to metal contact has much better adhesion in comparison with metal to oxide or oxide to oxide contacts. Therefore, even though this activation method is helpful, it is still limited in that the application of an alternating voltage is dependant on other conditions for performing the treatment of such as a comparatively small treating current and a comparatively high treating temperature.
U.S. Pat. No. 6,562,212 to Katafuchi et al. teaches a gas sensor element that is to be used in an oxygen sensor. In order to activate the electrode, the target gas electrode is first exposed to a reducing gas atmosphere at an elevated temperature of 400-900 C. Preferably, the temperature is kept at 700 C. The electrode is then connected to a voltage source circuit wherein AC voltage having the frequency greater than 1 Hz and amplitude between +/−0.1 and +/−5.0V is applied for 30 minutes. The reference fails to teach whether the voltage is applied during the time when the electrode is heating up to 700 C, or after the soaking period at that temperature. The present method can be distinguished from Katafuchi et al. in that square wave voltage of alternating polarity is applied to the electrode during the soaking period at a constant temperature, as well during the period when the electrode is being heated up or cooled down from a constant temperature.
Consequently, a need has been felt for providing a method for improving a platinum-zirconia oxygen sensor by improving platinum adhesion, increasing sensor electrode microporosity under treatment conditions that limit oxide formation at the platinum-zirconia interface.